Not applicable.
Not applicable.
1. Field of the Invention
The present invention generally relates to controlling the impedance of a printed circuit board trace. More particularly, the present invention relates to a printed circuit board that has a trace coupled to a slotted ground plane. The slotted ground plane includes slots incorporated in it, such that the impedance of the trace is increased.
2. Background of the Invention
Printed circuit boards are used in computers and other electronic applications to carry signals between electronic components. A printed circuit board typically includes layers of conductive material separated by layers of dielectric material. The conductive material may be a continuous plane or may be etched to remove material and leave, for example, traces for carrying signal. Multi-layer circuit boards typically include dedicated power, ground, and signal layers. Power layers receive outside power and transmit the power to the components. Ground layers provide ground both to components and to traces on signal layers. Signal layers include traces that may carry signal between any of a component, connector, cable, or other device mounted on the circuit board. Traces on different layers may be connected within the circuit board to another layer or to a component using a via. A via is a hole in a printed circuit board that is plated with conductive material. Thus, traces may transmit signals throughout the printed circuit board.
A signal is typically transmitted in the form of a change in voltage with time. The voltage may change from a low value to a high value or from a high value to a low value. Thus, the signal rises or falls. Typically, the time to effect a change in voltage is quantified in terms of the rise time, which is the time to rise from 10% to 90% of the maximum value of the voltage. The rapidness of the rise or fall indicates the transition speed of the signal. The term high speed signal refers to the transition speed, which typically differs from the speed of propagation of the signal, such as along the length of a trace.
A signal may alternately be described in terms of frequency or wavelength. A pulsed signal can be decomposed into a sum of oscillatory waveforms having different frequencies. The spread in frequency is termed the bandwidth of the signal. Conventionally, the bandwidth is about one third of the inverse of the rise time. Although the bandwidth is given in units of frequency, it is typically not the same as a fundamental frequency, for example of a clock, underlying the signal generation. The bandwidth of a signal defines an effective wavelength that is the wavelength of a hypothetical waveform propagating with a frequency equal to the bandwidth. A signal for which the length of the interconnect over which the signal is carried is at least about {fraction (1/7)} of the effective wavelength is conventionally termed a high speed signal.
Rise times for many devices, in particular those in consumer electronics, are now sufficiently short, such as 0.5 ns or less, that the rise time of the signal bandwidth is comparable to, or smaller than, the propagation time, the time it takes the signal to propagate the length of the interconnection, including any traces and vias. Therefore, wave effects of the signal are an important design consideration and the trace is treated as part of a high speed transmission line. For example, the transmission line impedance is an important consideration in the design of high speed circuit boards. The impedance determines the correlation between current and voltage. A mismatch in impedance at a location along a transmission line causes part of a signal to be reflected at that location. Depending on the amplitude and timing of the reflection, it may degrade the signal. In particular, it is desirable to reduce the amplitude and persistence of a reflection. Thus, it is the usual practice in high speed circuit board design to control the impedance of the traces so as to enhance the fidelity of signals transmitted through the traces.
Prior trace designs and methods of controlling the impedance of the traces have relied on known formulas or algorithms for the impedance for standard transmission line geometries, such as the microstrip and stripline geometries. A microstrip includes a trace with a rectangular cross-section separated from a solid ground plane by a dielectric material. The impedance of a microstrip increases with increasing height of the trace from the ground plane and decreases with increasing thickness and width of the trace, as well as decreasing with increasing permittivity of the dielectric material. Similarly, a stripline includes a trace with a rectangular cross-section between two parallel solid ground planes, separated from each ground plane by dielectric material. The impedance of a stripline similarly increases with the height from each ground plane and decreases with increasing trace thickness, trace width, and dielectric permittivity. However the specific dependence of the impedance on these parameters differs from that for a microstrip. In similar fashion, relationships are known for other various standard geometries. A designer may consult industry publications or use any of a number of computer programs, either freely available on the internet or distributed commercially.
With the progress of technology towards smaller computers and components, multi-layer printed circuit boards are becoming progressively thinner. In a thinner board, the distance of signal traces from adjacent ground planes must be decreased in order to maintain the same number of layers. Decreasing this distance lowers the impedance in the absence of other changes. One method of countering this affect is to decrease the trace width. However, as trace width decreases, the required tolerance on the width becomes smaller and more difficult to maintain. For trace widths below about 4 mils, it is very difficult to sufficiently control the trace width. Further, with decreasing width, DC resistance and signal attenuation tend to increasingly distort the signal. The above described conventional transmission line designs and methods of controlling impedance have the disadvantage that they fail to maintain a sufficiently high impedance of the trace to match the desired impedance, as multi-layer printed circuit boards become increasingly thin.
A particular difficulty arises in designing for the transmission of signals with different impedance values through different traces on the same layer of a printed circuit board. As computers become smaller and boards become thinner, it is desirable to be able to have the flexibility to use layers that are not dedicated to different values of the impedance. For example, it is desirable to provide a printed circuit board for use as a computer back plane that is able to transmit both logic signals at a base impedance of about 50 Ohms and SCSI signals at an increased impedance of about 100 to about 120 Ohms. The height between the ground plane and the respective traces is the same for signals on the same layer, so the height cannot be varied to vary the impedance of the respective traces. Further, with current limits on trace width, the required 100-120 Ohm impedance cannot be achieved for thin layers with conventional microstrip or stripline geometries and conventional FR4 dielectric material.
Thus, it would be desirable if a system and method were available that provided high speed, high impedance signal transmission in thin multi-layer printed circuit boards.
The present invention solves the deficiencies of the prior art by providing a configuration for a printed circuit board circuit that incorporates a slotted ground plane.
According to one preferred embodiment, a printed circuit board includes a signal trace for transmitting an electrical signal, and a ground plane coupled to the trace, where the ground plane includes a slot through it in the vicinity of the trace. Further, the slot is coupled to the trace with a distributed inductance. The slot creates an inductive effect that raises the impedance. The printed circuit board may include a second trace coupled to the first trace. Further, the printed circuit board may include a second ground plane having another plurality of slots through it in the vicinity of the trace or traces, where the slot is coupled to the trace, or traces, with a distributed inductance. Preferably, the impedance of each trace is set at a predetermined value. The printed circuit board may further include a second trace substantially uncoupled from the slot, where an impedance of the first trace is set at a predetermined value and an impedance of the second trace is set at a second predetermined value lower than the first value.
According to another preferred embodiment, a printed circuit board includes a conductive layer that includes a transmission line of at least one strip for transmitting an electrical signal having a characteristic wavelength, a dielectric layer adjacent the conductive layer, and a second conductive layer adjacent the dielectric layer. The second conductive layer includes an array of windows through it, the array including at least two substantially identical subarrays. The subarrays are preferably displaced by a repetition distance substantially less than the distance the signal propagates in an amount of time equal to the rise time of the signal. The subarrays may be symmetrically disposed transverse to the transmission line. A subarray may include one window, two windows, or more. Further, the transmission line may include a single conductive strip or a pair of coupled conductive strips. Still further, the printed circuit board may include another dielectric layer adjacent the first conductive layer and yet a third conductive layer adjacent the second dielectric layer, where the third conductive layer includes another array of windows that extends through it. The second array may be an inverse image of the first. Alternatively, the second array may be a mirror image of the first.
According to still another preferred embodiment, a printed circuit board includes a conductive strip and a conductive planar layer coupled to the strip, where the planar layer includes at least two voids through it. The planar layer further includes at least one subplane between the voids, the subplane providing a spacing tangential to said strip. Each of the widths and the length are set at a predetermined value such that the impedance of the trace is set at a predetermined impedance. The circuit board is incorporated in a computer. The voids may have a polygonal shape, such as square or rectangular. Alternatively, the voids may have an arcuate shape, such as circular or ellipsoidal. Still alternatively, the voids may have a combined polygonal and arcuate shape.